Conventionally in electrostatic photography, an electrostatic latent image has been formed on a photoconductive plate such as selenium, zinc oxide or the like, which has been electrostatically developed by adding developer particles comprising carrier particles such as glass beads, iron powder or the like and colored micropowder of insulating toner charged by friction, contacting with carrier particles.
Such developed image has been directly recorded or transferred by piling a transference sheet thereon and applying an electric field.
The images transferred on sheets have been fixed, for example, by pressure or heat.
On the other hand, in order to simplify and miniaturize electronic copying apparatus, magnetic toners employing no carrier have been developed. There has been proposed a process in which toners contain magnetic powder, such as magnetite (Fe.sub.3 O.sub.4), provided with high electroconductivity and the development is carried out by inducing the reverse charge to the electrostatic image (U.S. Pat. No. 3,639,245). There have been found magnetic toners usable for such developing system (U.S. Pat. No. 3,925,219). Such electroconductive magnetic toners are mainly used for the direct recording on the photosensitive body, frequently on the photosensitive paper containing zinc oxide. By the use of such toners and recording system, highly distinct copies can be provided by means of a copying apparatus having a simple structure. The copying system which has been conventionally desired is not of a system recording directly on photosensitive paper but of a system recording on normal recording paper or plain paper, i.e., a system in which toner particles adhering on a photosensitive body by the development process are transferred to a transferring sheet by piling the toner particles on the photosensitive body onto the transferring sheet and applying electric field. By such a system, ordinary paper can be usable as the transferring sheet and thus has an advantage that copies can be obtained without employing expensive photosensitive paper.
For such a transferring system, there have been conventionally used binary developer powder in which carriers and toners are mixed together. If the above-mentioned magnetic toners are employed, copying apparatus can be simplified and miniaturized. Additionally substantial merit can be expected because there is no requirement for controlling the mixing ratio of the carrier and toner, the absence of deterioration of the developing agent due to deterioration of the carrier, and the absence of liberation of carrier waste. For such reasons, there has been proposed an indirect recording system, in which the developed image has been fixed on a transferring sheet by employing such electrically conductive magnetic toners (Japan Laid Opened Patent Application No. 26044/1976). However, as toners are electrically conductive, the transferrence of the toners cannot be carried out well, even if we modify the transferring system. It was difficult to provide copies usable in practical use.
Hence, there have been proposed developing and transferring systems in which the electro-conducivity of magnetic toners is reduced to a value of less than 10.sup.-6 .multidot.cm.sup.-1 to provide insulating toners such as conventional non-magnetic toners and the inner polarization in the toners by electric field is utilized (Japan Laid Opened Patent Application No. 90336/1975, 92137/1975 and 133026/1976). In addition, there have been proposed, in the use of insulating magnetic toners, a transferring process in which charge of the same polarity as, or inverse polarity to, the electrostatic image is applied on the toner on the photosensitive body and the image is transferred by charge of the same polarity as, or inverse polarity to, the electrostatic image (Japan Laid Opened Patent Application No. 102644/1976), and such a process in which the image is transferred to transferring sheet which has been charged (Japan Laid Opened Patent Application No. 72436/1976). In addition, there have been proposed various toners comprising a thermoplastic resin and magnetic fine particles and having a electro-conductivity of less than 10.sup.-3 .multidot.cm.sup.-1 provided with a fluidity improver (Japan Laid Opened Patent Application No. 101535/1976, 126836/1976 and 133028/1976).
The inventors have found that there arise various problems frequently when images are developed by conventionally known developing processes and transferred from the photosensitive body by employing conventionally known toners and that such problems result from the toners being used so that care should be taken in the electric characteristics such as electric conductivity, dielectric constant, etc. of toners, and particle size of toners.
It is important to take particular consideration of resin composition of toners corresponding to the fixing procedure of the toners to the transferring sheet.
Using conventional toners which have not taken into account such conditions may cause lack of uniformity of a developed image, poor transferring efficiency, or fogging, or tendencies to cause fogging, irregular transferrring, and surface roughness of image when toners of high transferring efficiency are used. Hence such toners cannot be considered to be practical.